JP2000192201A - High corrosion resistance austenitic stainless steel for heat transfer pipe for waste incinerating plant boiler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide austenitic stainless steel for a boiler heat transfer pipe having excellent resistance to molten salt corrosion and acid dew point corrosion in a waste incinerating power generating plant and having excellent intergranular corrosion resistance in molten salt corrosive environments in particular. SOLUTION: This steel is the one having a compsn. contg., by weight, <=0.05% C, 2.0 to 4.0% Si, >0.5 to 2.0% Mn, 20 to <25% Ni, 20 to 30% Cr and <=0.15% N, also simultaneously satisfying the following formulas I and II, and the balance substantial Fe and inevitable impurities. If required, the steel can be added with <1.0% Mo, one or two kinds of Nb and Ti by 0.1 to 1.5% in total and 0.001 to 0.015% Ca individually or combinedly: the formula I: Si(%)×Cr(%)-15.2)>=17.6 and the formula II: Cr(%)+6×Si(%)<=48.0%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to waste (so-called garbage) such as municipal waste and industrial waste,
The present invention relates to a high corrosion resistant austenitic stainless steel used as a boiler heat transfer tube such as a boiler evaporator tube, a water wall tube, and a superheater tube installed in a facility that generates power using DF (Refuse Derived Fuel) as a raw material.

[0002]

2. Description of the Related Art In recent years, there has been a movement to reduce CO ₂ emissions by utilizing energy that has been left unutilized until now to achieve high efficiency. Has begun to spread. In Japan, the recovery and effective use of waste energy is being actively promoted, and in the old days, it was only used to supply electricity in incineration facilities and district heating, but at present, advanced (Highly efficient) ones.

However, in order to efficiently recover waste energy as electric energy, the steam temperature and pressure of a boiler generally need to be increased.
The high efficiency is accompanied by the highly corrosive waste quality unique to Japan, and severe corrosion damage to the boiler heat transfer tube material due to the molten salt contained in the combustion ash and the combustion exhaust gas is inevitable. Therefore, it is recognized that the material life of the heat transfer tubes for waste power generation boilers largely depends on the corrosion resistance at high temperatures, and although various corrosion-resistant materials have already been proposed, they have been successfully demonstrated at metal temperatures of 400 ° C or higher. There are still a few examples.

[0004] Thus, in view of the thermal recycling situation in Japan, as materials for heat transfer tubes of waste power boilers, materials that can withstand severe high-temperature corrosive environment due to combustion ash and exhaust gas and acid dew point corrosive environment generated when the furnace is shut down. The emergence is much desired and the development and demonstration of such materials is urgent. Japanese Unexamined Patent Publication No. 4-35014 which has already improved corrosion resistance and stress corrosion resistance for use in waste incineration waste heat boiler tubes.
No. 9 has been proposed. However, this is because Ni
Is less than 25%, the addition of Si is considered to have an adverse effect on high-temperature corrosion resistance, and is a high-cost material due to the addition of a large amount of Ni. However, as a result of further detailed experiments by the present inventors, the addition of Si is effective for high-temperature corrosion resistance if the above-mentioned formula (1) is satisfied even in a component system of Ni: 25% or less. It was found that the material of the present invention had excellent properties as a heat transfer tube for a waste power generation boiler.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to increase the efficiency of waste incineration power generation in a severe corrosive environment in Japan and extend the life of a boiler by using high-temperature corrosive gas, molten salt, and the like. An object of the present invention is to provide a boiler heat transfer tube material for waste incineration power generation having excellent corrosion resistance in a severe corrosive environment in which an acid or the like exists alone or in combination.

[0006]

SUMMARY OF THE INVENTION The gist of the present invention is a highly corrosion-resistant austenitic stainless steel for a boiler heat transfer tube of a waste incineration plant having the following chemical composition. The main points are as follows: (1) By weight%, C: 0.05% or less, Si: 2.0 to
4.0%, Mn: more than 0.5 to 2.0%, Ni: 20 to 2
Less than 5%, Cr: 20-30%, N: 0.15% or less, and simultaneously satisfying the following expressions (1) and (2),
A high corrosion resistant austenitic stainless steel for a boiler heat transfer tube of a waste incineration plant, the balance substantially consisting of Fe and unavoidable impurities. Si (%) × {Cr (%) − 15.2} ≧ 17.6% (1) Cr (%) + 6 × Si (%) ≦ 48.0% (2)

(2) A high corrosion resistant austenitic stainless steel for a waste incineration plant boiler heat transfer tube, characterized by further containing Mo: less than 1.0% by weight in addition to the alloy component described in the above (1). . (3) In addition to the alloy components described in the above (1) or (2), further, any one of Nb or Ti or 2
A high-corrosion-resistant austenitic stainless steel for a boiler heat transfer tube of a waste incineration plant, comprising 0.1 to 1.5% by weight of seeds in total. (4) A boiler heat transfer tube for a waste incineration plant, characterized by further containing Ca: 0.001 to 0.015% by weight in addition to any of the alloy components described in (1) to (3) above. For high corrosion resistance austenitic stainless steel.

The technical concept and features of the present invention are as follows. (1) Resistance to molten salt corrosion and acid dew point corrosion is achieved by simultaneous formation of Cr ₂ O ₃ and SiO ₂ coatings on the leading edge of the alloy. (2) Deterioration of corrosion resistance due to formation of a Cr deficient layer generated by precipitation of grain boundary Cr carbide is compensated for by formation of a SiO ₂ film. (3) By reducing the C content, grain boundary erosion due to the formation of a Cr-deficient layer is suppressed. (4) C inevitably contained is fixed by Nb and Ti, and grain boundary erosion due to formation of a Cr-deficient layer is suppressed. Based on the technical idea described above, it is a feature of the present invention that the molten salt corrosion resistance, particularly the intergranular corrosion resistance is improved by the combined addition of Si and Cr (relationship of the formula (1)). Furthermore, it has been found that one of the features of the present invention is that efficient production can be performed when the chemical composition satisfies the relationship of the formula (2).

The function of each alloy component of the steel of the present invention and the reason for limiting the content thereof will be described below. C: 0.05% or less A large amount of C causes precipitation of a large amount of Cr carbide at a crystal grain boundary at a high temperature and promotes the progress of intergranular corrosion. Therefore, it is desirable that the content is reduced as much as possible. C content is 0.
If the content exceeds 0.05%, the above-described problem becomes noticeable, so the content is set to 0.05% or less.

Si: 2.0 to 4.0% Si is an element generally used for the purpose of a deoxidizing agent.
In the present invention, as described above, it is used for imparting excellent corrosion resistance to the material by simultaneous addition with Cr. Since this effect becomes significant when the Si content is 2.0% or more, the lower limit is set to 2.0%. However, when the Si content exceeds 4.0%, the hot workability deteriorates remarkably, so the upper limit was set to 4.0%.

Mn: more than 0.5 to 2.0% Mn is added to deoxidize and stabilize the austenite structure. If the content is 0.5% or less, there is no deoxidizing effect, and if the content exceeds 2.0%, the hot workability deteriorates. Therefore, the content is set to 2.0% or less. Ni: less than 20 to 25% Ni is added in order to stabilize the austenite structure and secure sufficient mechanical properties. In order to ensure the stability and mechanical properties of the austenitic structure, the content is required to be less than 20 to 25%.

Cr: 20 to 30% Cr is an element indispensable for improving the molten salt corrosion resistance and acid dew point corrosion resistance of the material. In particular, with respect to the molten salt corrosion resistance, the effect is more exerted by adding Si in combination with Si. Since the effect is recognized when the content is 20% or more, the lower limit is set to 20%. On the other hand, if the content exceeds 30%, it becomes difficult to maintain the austenite structure in holding at a high temperature, and it is easy to cause embrittlement. Therefore, the upper limit is set to 30%.

N: 0.15% or less N is an element that not only contributes to stabilization of the austenite structure, but also has the effect of increasing the high-temperature strength, but as the content increases, hot and cold workability increases. Therefore, the upper limit is set to 0.15%, and desirably is set to more than 0.04%. In addition to the components described above, in the present invention, in addition to the chemical composition described above, if necessary,
Mo can be contained.

Mo: less than 1.0% Mo is an element that not only improves high-temperature strength but also exhibits excellent acid dew point corrosion resistance. Particularly when these effects are desired, they can be added as necessary. However, M
Since the hot workability tends to decrease as the o content increases, the upper limit is set to less than 1.0%. further,
In the present invention, in addition to the chemical composition described above, Nb
Alternatively, one or two of Ti can be selected and contained.

Nb, Ti: 0.1 to 1.5% in total of one or two of Nb and Ti. Since Nb and Ti both easily form carbides,
By fixing C in the steel and preventing the generation of a Cr-deficient layer, it has an effect of preventing deterioration of intergranular corrosion resistance and resistance to acid dew point corrosion in a molten salt corrosion environment. To obtain these effects, if necessary,
And Ti are selectively contained. In this case, if the total is less than 0.1%, no effect is recognized, and if it exceeds 1.5%, the hot and cold working is significantly deteriorated. .1 to 1.5
%.

Further, in the present invention, Ca may be contained in addition to the above-mentioned chemical composition. Ca: 0.001 to 0.015% Since Ca has an effect of fixing S, which is one of the factors that deteriorate hot workability, it is added when the purpose of the present invention is to improve hot workability. can do. When it is contained, it is desirable to add 0.001% or more.
If the addition exceeds 015%, a Ca—Ni compound having a low melting point is formed, and conversely, hot workability is deteriorated.
015%.

Si (%) × {Cr (%) − 15.2} ≧ 17.6% (1) As described above, as a result of repeated experiments by the present inventors, in the waste incineration environment, The effect on molten salt corrosion is significantly increased when Si and Cr are properly added, and when the content of Si and Cr satisfies the condition of formula (1), the molten salt corrosion resistance, particularly the intergranular corrosion resistance It was found that the properties improved synergistically. Cr (%) + 6 × Si (%) ≦ 48.0% (2) However, as the value of the formula (1) increases, the molten salt corrosion resistance improves, but the value of the formula (2) is 48%
, The hot workability is remarkably deteriorated, so that the complex content of Si and Cr is limited to 48.0% or less as in the equation (2).

[0018]

EXAMPLES As examples, Table 1 shows the chemical compositions of the steels of the present invention, comparative steels and existing steels. Adopted three steel grades as existing steel,
Existing steel Nos. 38, 39 and 40 are SUS310
S, SUS316, and NCF800H. Alloy of the chemical composition shown in Table 1 was melted in a vacuum melting furnace at 100k
Each of the obtained ingots was hot forged into a plate having a width of 60 mm and a thickness of 10 mm, and then subjected to a product heat treatment at 1160 ° C. Then, through the machining and grinding process,
A corrosion test specimen of 0 mm × 10 mm × 3.5 mm was prepared.

[0019]

[Table 1]

The evaluation of molten salt corrosion resistance was performed by the burial method. In the experiment, using a crucible having an outer diameter of 26 mm × a height of 19 mm and a capacity of 5 ml, the test piece was buried in the attached ash (adjusted to 150 μm or less) collected by a waste incineration boiler, and the incinerator was corroded. 10% CO ₂ -10 simulating anaerobic gas
% O ₂ -50 ppm SO ₂ -50 ppm NO-100p
pmCO-1500 ppm HCl-20% H ₂ O-ba
l. This was carried out in a process of maintaining the temperature of 550 ° C. for 100 hours while flowing a mixed gas of N ₂ at a flow rate of 600 ml / min. The ash used in the experiment had the composition shown in Table 2. After the experiment, descaling was performed, and the weight loss of the sample was measured. Thereafter, the sample was cut in the thickness direction, and the maximum overall corrosion depth and the maximum intergranular corrosion length were measured and evaluated using an optical microscope.

[0021]

[Table 2]

The acid dew point corrosion resistance is determined by applying a real furnace ash to a sample,
After holding for 100 hours in a steam-saturated 80 ° C. thermostat,
Evaluation was made by confirming the occurrence of local corrosion with an optical microscope and measuring and comparing the depth. Table 3 and FIG. 1 show the results of these experiments. In addition, about comparative steel No. 36 and No. 37, it was not possible to perform an experiment because a remarkable forging crack was caused at the time of hot forging of a steel ingot and a test piece could not be adjusted.

As a result of analyzing the experimental results, as shown in FIG. 1, in the component range specified by the present invention, the molten salt corrosion resistance is improved by the synergistic effect of Si and Cr (formula (1)). It was clear that it would be organized. further,
The maximum general corrosion depth of the steel of the present invention is up to 1/5 that of the existing steel.
Since the maximum length of grain boundary erosion is less than 0.01 mm in any of the examples, the steel of the present invention has a molten salt corrosion resistance of up to 4 to 5 times that of the existing steel. It was confirmed to have. In an environment in which intergranular corrosion is concerned in a high-temperature environment, for example, invention steels No. 18 and No. 1
As is clear from the comparison between No. 9 and No. 22 and No. 23, N
It can be seen that the addition of b and Ti alone or in combination is effective.

The steel of the present invention is also superior in acid dew point corrosion resistance to comparative steels and existing steels.
The maximum pit depth was less than m. In particular, in a composition containing a large amount of Cr and Mo, for example, as shown by invention steels No. 1, No. 16, and No. 17, pitting corrosion was not observed. Alternatively, selective addition of Mo is desirable.

[0025]

[Table 3]

[0026]

As described above, the steel of the present invention has excellent molten salt corrosion resistance even in an extremely severe corrosive environment such as a waste incineration environment, and is resistant to acid dew point corrosion that occurs when the furnace is shut down. Austenitic stainless steel having high resistance was also shown. Therefore, when a pipe made of the steel of the present invention is applied to a high-temperature portion of a boiler such as a superheater pipe, the temperature and pressure of a waste incineration boiler can be increased, and the heat of incineration waste heat in accordance with thermal recycling can be obtained. Highly efficient utilization becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing a synergistic effect of Si and Cr on weight loss due to corrosion. Patent applicant Sanyo Special Steel Co., Ltd. and 10 others Attorney Patent Attorney Shiina Jin

 ──────────────────────────────────────────────────続 き Continuation of front page (71) Applicant 000133032 Takuma Co., Ltd. 1-3-3 Dojimahama, Kita-ku, Osaka-shi, Osaka (71) Applicant 000004123 Nippon Kokan Co., Ltd. 1-1-2 Marunouchi, Chiyoda-ku, Tokyo, Japan (71) Applicant 000006208 Mitsubishi Heavy Industries, Ltd. 2-5-1 Marunouchi, Chiyoda-ku, Tokyo (71) Applicant 000000239 Ebara Corporation 11-1 Asahicho, Haneda, Ota-ku, Tokyo (71) Applicant 000000099 Harima Ishikawajima Heavy Industries, Ltd. 2-2-1, Otemachi, Chiyoda-ku, Tokyo (71) Applicant 000001052 Kubota Corporation 2-47, Shikitsu-Higashi 1-chome, Naniwa-ku, Osaka-shi, Osaka (71) Applicant 000001199 Kobe Steel, Ltd. 1-3-18 Wakihama-cho, Chuo-ku, Kobe-shi, Hyogo (71) Applicant 000180070 Sanyo Special Steel Co., Ltd.Shimima-ku, Himeji-shi, Hyogo Sanji One Letter 3007 (72) Inventor Atsushi Sho 3007 Nakajima One Letter in Shima, Himeji City, Hyogo Prefecture Inside Sanyo Special Steel Co., Ltd. (72) Inventor Tatsuro Isomoto 3007 1 character, Nakajima, Shima, Himeji-shi, Hyogo Sanyo Special Steel Co., Ltd. (72) Inventor Takeo Ubube 2-chome, Aomi, Koto-ku, Tokyo Tokyo Cleaning Bureau Chubu Inside the government building (72) Inventor Kimiaki Nagashima 2-8-1, Nishi-Shinjuku, Shinjuku-ku, Tokyo Inside the Tokyo Metropolitan Government Cleaning Department Plant Management Department (72) Inventor Sadao Suzuki 2-2-1, Nishi-Shinjuku, Shinjuku-ku, Tokyo Bureau factory management department

Claims (4)

[Claims] 1. wt%, C: 0.05% or less, Si: 2.0 to 4.0%, Mn: more than 0.5 to 2.0%, Ni: 20 to less than 25%, Cr: 20 to 30%, N: 0.15% or less, and simultaneously satisfying the following formulas (1) and (2), with the balance substantially consisting of Fe and unavoidable impurities: High corrosion resistant austenitic stainless steel for heat transfer tubes in incineration plant boilers. Si (%) × {Cr (%) − 15.2} ≧ 17.6% (1) Cr (%) + 6 × Si (%) ≦ 48.0% (2) 2. In addition to the alloy component according to claim 1,
Further, Mo: a highly corrosion-resistant austenitic stainless steel for a waste incineration plant boiler heat transfer tube, characterized by containing less than 1.0% by weight. 3. An alloy according to claim 1, further comprising one or two of Nb and Ti in a total amount of 0.1 to 1.5% by weight. High austenitic stainless steel for waste heat incineration plant boiler heat transfer tubes. 4. In addition to any of the alloy components according to claim 1, Ca: 0.001 to 0.015
High corrosion-resistant austenitic stainless steel for boiler heat transfer tubes in a waste incineration plant characterized by containing by weight.